The development of thin-film transistors employing an organic semiconductor gradually began to become more active in the latter half of the 1980s, and has in recent years reached the stage where the characteristics of a thin-film transistor employing an organic semiconductor exceed the characteristics of a thin-film transistor of amorphous silicon for basic performance. Organic materials are easily processed and generally have a high affinity with a plastic substrate on which a thin-film field effect transistor (FET) is usually formed. Therefore, the use of an organic material as a semiconductor layer in a thin-film device is desirable. The organic semiconductor include acenes such as pentacene and tetracene, disclosed by Japanese Patent Application Laid-Open No. H5-55568; phthalocyanines including lead phthalocyanine, and low-molecular-weight compounds such as perylene and tetracarboxylic acid derivatives thereof, disclosed by Japanese Patent Application Laid-Open No. H5-190877; and aromatic oligomers typified by thiophene hexamers referred to as α-thienyl or sexithiophene, as well as high-molecular-weight compounds such as polythiophene, polythienylenevinylene and poly-p-phenylenevinylene, disclosed by Japanese Patent Application Laid-Open No. H8-264805. (Many of these compounds are also disclosed by “Advanced Materials”, Vol. 2, p. 99, 2002.)
Properties such as non-linear optical properties, electrical conductivity and semiconductivity, which are required for using the above compounds as the semiconductor layer in a device, depend not only on the purity of the compounds but also largely on crystallinity and orientation of the compounds. CuKα X-ray diffraction of a vapor-deposited film of a compound having an extended π-conjugation, for example pentacene, has been reported in terms of Bragg angle 2θ (Japanese Patent Application Laid-Open No. 2001-94107). In addition to this report, there are many other reports of field effect transistors that use a vapor-deposited film of pentacene in the semiconductor layer, whereby it is known that the transistors show high field-effect mobility because of the excellent crystallinity and orientation of the vapor-deposited film of pentacene. However, because pentacene is unstable in air and tends to be oxidized and easily degraded, an apparatus for forming a film of pentacene by vacuum deposition was required, which is a problem of increasing the costs of the transistors using a film of pentacene.
On the other hand, a field effect transistor has also been reported which was produced by a method of coating with a low-molecular-weight compound and converting the compound to an organic semiconductor compound by heating. In this method, although tetrachlorobenzene is eliminated by the heating, since the boiling point of tetrachlorobenzene is high, it remains in the system at normal pressure, thus hindering the crystallinity of the pentacene (J. Appl. Phys. Vol. 79, p. 2136, 1996). In addition, a method of converting to pentacene by the elimination of ethylene has been reported, and this method was pointed out that the elimination of ethylene is difficult to carry out and therefore a precursor in this case is unsuitable (“Advanced Materials”, Vol. 11, p. 480, 1999).
It has also been reported that it is possible to utilize tetrabenzoporphyrin obtained by heating at 210° C. a porphyrin formed by ring-shrinking a bulky bicyclo[2.2.2]octadiene skeleton as an organic semiconductor (Proceedings of the 81st Annual Spring Meeting of the Chemical Society of Japan, 2002, II, p. 990, 2F9-14; Japanese Patent Application Laid-Open No. 2003-304014; Japanese Patent Application Laid-Open No. 2004-6750). However, in this case it is thought that optimum crystal orientation for carrier movement is not achieved, whereby sufficient characteristics were unable to be obtained.
As described above, conventionally field effect transistors employing an organic semiconductor compound have been produced with a semiconductor layer having crystallinity and orientation which is formed by a process such as vacuum film formation. However, as the typical example, acenes have the problem of being likely to be degraded by oxidation. Also, the above conventional simple technique employing a coating method has a problem of not producing a film with crystallinity and orientation showing sufficient characteristics.